Process for producing stainless steel



Patented Feb. 6, 199233.

Unit stars FRANK n. cannnr, or new rank, n. Y.

PROCESS FOR PRODUCING STAINLESS STEEL.

No Drawing.

To all whom it may concern Be it known that I, FRANK D. CARNEY, a citizen of the United States of America, and a resident. of New York, county and State of New York, U. S. A., have invented certain new and useful Improvements in.

Processes for Producing Stainless Steel;

and I do hereby declare the following to beso low in carbon that it can be readily rolled I in rolling mills for subsequent forming processes, such as deep drawing, corrugatin stamping, &c., whereby wire, sheets, har ware, tubes, kitchen utensils and many other finished roducts may be made of stainless and rust ess steel.

I can therefore make from such' a steel finished products which it is not practicable to make by present methods by reason of the prohibitive costs.

In order to give to a steel stainless properties, it is customary to have from about seven to twenty per cent chromium in the steel, but the carbon content is too high for the usual manufacture. of commercial steel articles such as mentioned above.

The chromium content of stainless steel is given to it by the addition to the melted steel of ferro-chromium, the cost of which increases with the decrease in the carbon content of the ferro-chro-mium, the cheaper grades of which carry from about seven to ten per cent carbon, and about sixty per cent chromium.

It is the effect of this undesirable high carbon content of the ferro-chromie addition in the finished metal that prohibits such stainless steel from being used for purposes where tonnage would be involved, as for roofing, hardware, &c., and the methods now used are not adapted to tonnage production.

Stainless and rustless steel for cutlery and similar articles where the elastic limit is required to be high contains around 0.50% carbon.

For the purposes outlined by me the stainless steel must contain less than 0.15% carbon, and preferably less than 0.10% to Application filed January 24, 1922. Serial No. 531,464.

readily adapt it to forming for the purposes speclfied.

Now, in order to show how the ferrochrome addition to steel raises the carbon content in the finished metal I shall give a few examples. I

In order to add twelve per cent chromium to the steel by means of ferro-chromium containing chromium, it would require a 20% addition to the steel of ferro-chromium containing about 10% carbon. This would make a, carbon addition to the steel of 2.00%.

If the carbon content of the ferro-chrome were low, say 7%, the carbon addition would be 1.40%, which is yet too high for the purposes intended.

The problem then is to get rid of the carbon without .unduly sacrificing the chromium. Chromium is prone to oxidize under the usual steelmaking conditions.

Now, inaccordance with my invention I eliminate the carbon after the ferro-chrome addition by brin ing the metal to a very hot condition, an passing air through this unusually hot metal.

At the usual steel making temperatures chromium will oxidizewhen blown before the carbon, but when the temperature of the metal is high enough a reversal of the afiinities of carbon and chromium for oxygen takes place, and the carbon will oxidize first.

A hot or high temperature metal, preparatory to oxidizing may be obtained in several ways, but I prefer to do this by an addition of ferro-silicon to the metal, as it appears to be the cheapest, needs no special apparatus, and may be practised in any Bessemer plant. Besides this, most furnace men have been accustomed to use ferro-silicon in other operations.

As illustrative ofthe specific manner in which my process may be carried out, I place in a melting furnace a charge of steel,

preferably very low carbon steel having.

about 0.10% to 0.20% carbon, and add sufii cient ferro-ohrome to the charge to make the final analysis of'the metal within the limits of chromium content required for the particular stainless steel to be made.

This addition will raise the carbon content of the bath in the neighborhood of 1.50%.

Ferro-silicon is then added to this bath in such quantity that the silicon content of the mixture is about 2.00% and the mixture is then transferredto a Bessemer converter,

electric twyer furnace, or equivalent furnace, and blown.

Silicon is the first metal oxidized in the Bessemer process and at the same time generates very rapidly sufficient heat to raise the temperature of'the bath above the normal temperature at which metal is treated in converters. When the temperature reaches a certain point the aflinities of silicon and carbon for oxygen reverse, and silicon will no longer be oxidized but the carbon will be. At thishigh temperature chromium. is also not readily oxidized, so that the oxidation of the carbon will go on in preference to the oxidation of the chromium.

If the temperature should drop during the blowing, silicon again begins to oxidize in preference to carbon and thus heats the metal, the silicon thus acting as an element to control the temperature.

After the carbon has been reduced to the point desired, say below 0.15% or 0.10% the converter is lowered and the metal poured into a ladle; or if an electric furnace provided with twyers has been used, it is tapped in the usual way into the ladle, with the customary addition of ferro-manganese or any other finishing metal. Or the metal may be transferred to the usual electric furnace for more complete de-oxidation..

The silicon content of the mixture charged into the Bessemer converter acts therefore as a controller for the temperature, keeping the metal up to its high temperature by the oxidation thereof until the carbon has been reduced to the limits desired, say below o.15%,

Naturally the quantity of carbon left in the blown metal will depend upon the quan tity of silicon contained in the unblown metal, and this silicon content must be proportioned according to the carbon elimination desired and also in accordance with the customary conditions found in the particular plant, and which the furnace manager takes into consideration, such as the temperature of the converter or other furnace at the beginning of the blow, the temperature of the metal when charged into the furnace, the size of the converter, the depth of the bath, the air pressure, the air volume, &c., conditions which are well known and always taken into consideration when blowing metal.

I have found, however, that a silicon content of 2.00%, slightly more or less, will be suiiicientunder the usual furnace conditions to secure the low carbon content desired, at the same time preventing the oxidation, during blowing, of any prohibitive quantity of ties of oxygen for carbon and chromium and threby cause the'carbon to oxidize before the chromium.

2. The method of making stainless steel which comprises adding ferro-chrome and sufficient ferro-silicon to raise the temperature high enough to change the relative affinities of oxygen for carbon and chromium to a steel and blowing the melt so formed.

9 3. The method of making stainless steel which comprises adding to steel under oxidizing conditions ferro-chrome sufiicient to give the desired chromium content to the nished metal, and an element which oxidizes more readily than carbon under furnace conditions and upon oxidation causes a reversal of the afiinities of carbon and said element for exygen.

4. The method of making stainless steel which comprises adding to low carbon steel ferro-chrome sufiicient to ive the chromium content desired for the finished metal and ferro-silicon, and blowing the metal until the carbon is reduced to the desired low limit, and finishing the metal.

5. The method of making low carbon stainless steel which comprises melting low carbon steel, adding thereto ferrochrome in quantity sufiicient to give the desired chromium content for the finished metal and ferro-silicon, and blowing the metal to reduce the carbon below 0.15%.

6. The. method of making low carbon stainless steel Which comprises melting a steel containing carbon below 0.20% and adding thereto ferro-chrome, in quantity sufficient to give the desiredchroinium content for the finished metal, and ferro-silicon sufiicient to maintain a reversal of the affinities of carbon and silicon during the elimination of carbon to a low percentage.

7. The method of making low carbon stainless steel which comprises adding to steel below 0.20% carbon content, ferrochrome in quantity to give the desired chromium content for the finished metal and ferro-silicon not exceeding two per cent, and blowing the metal to eliminate carbon below 0.15%.

In testimony that I claim the foregoing as my invention, I have signed my name 

